This invention relates to an engine idling speed control apparatus arranged to suppress engine speed changes before and after application of an external load to the engine.
It is the current practice to maintain the engine speed at or near a target value by controlling the amount of air supplied to the engine, the amount of fuel metered to the engine and the ignition timing in a direction to reduce the deviation of the sensed engine speed from the target value. However, such a conventional engine idling speed control cannot avoid an abrupt engine idling speed change resulting from an abrupt engine load change when an air conditioner or other accessory is switched on or off.
It is proposed, in Japanese Patent Kokai No. 61-169642 to avoid such an abrupt engine idling speed change by advancing the ignition timing from a basic value while increasing the amount of air supplied to the engine upon application of an external load to the engine. In order to achieve good response to the external load application, the basic ignition timing value should be set at a value deviated sufficiently from the value MBT to provide a sufficient margin for the torque increase resulting from the ignition timing advancement operation required upon the external load application. In order to achieve good fuel economy, the basic ignition timing value should be set near the value MBT. For this dilemma, it is difficult to achieve good response to an engine load change as well as good fuel economy.